During an intervention on an angiography system, for instance in order to navigate the instruments (e.g. in the head or heart), real-time images are obtained using fluoroscopy. An intervention that is frequently performed on such a system is the embolization of tumors or arteriovenous malformations (AVM) as shown in FIG. 1.
Arteriovenous malformations (AVM) are congenital malformations of the vascular system, often malformations of the vascular system of the central nervous system, the brain or the facial skeleton. In such a malformation there is a direct connection between the arteries and the veins of the vascular system. Hence between the arteries and the veins there are no capillary vessels in which the actual material transfer takes place between the blood and tissue. One consequence of this is that the tissue area concerned is not supplied with blood. Another consequence is that the pressure rises in the veins, which can cause them to enlarge and potentially result in hemorrhaging. In particular, brain hemorrhages may be highly critical here.
There are three techniques available today for treating AVM, which are mostly also used in combination. These treatments are neurosurgical operations, radiation treatment and endovascular therapies. Whatever the type of AVM treatment, it is necessary to have precise knowledge of the position, shape and form of the AVM and of the detailed blood-flow conditions in order to plan and carry out the treatment. Hence both morphological information (position, shape and type of blood vessels) and functional time-dependent information (flow conditions) is required.
Computer tomography and magnetic resonance tomography in particular are possible as non-invasive imaging modalities for diagnosis. Often an angiography is also carried out in order to obtain precise information and plan treatment in detail. In this case there are two alternatives available: C-arm based imaging that is static in time and spatially resolved in three dimensions or imaging that is time-resolved in one dimension and spatially resolved in two dimensions.
Interventional endovascular therapy is performed in the angiography laboratory using fluoroscopy. Angiography views (in particular DSA views) through the relevant vascular region are produced for planning and checking. The views can be recorded on monoplane equipment, although biplane equipment, which records two views in parallel from different angulations, are more suitable. Analysis can be performed on each of the two views
It is important here to track the continuous progress of the embolization, particularly in order to prevent the embolic agent flowing back into unaffected vessels. Usually such procedures are monitored using subtracted frames, in which only the differences from a specific mask image are visible, as shown in illustration B) of FIG. 1) by way of example. One advantage of this is that anatomic backgrounds are “canceled out”. In addition, the progress of the embolization can be monitored more easily, because after the mask is re-initialized, the mask and also the embolic agent that has accumulated up until this instant is no longer visible in the (subtracted) frame sequences.
This can be a disadvantage, however, because after the mask is re-initialized, the doctor can no longer identify the areas that have already been embolized. It is also advantageous for the doctor not to see the newly embolized areas as a “growing black region”. What is important to the doctor is to be able to assess the progress, i.e. identify at a glance which area has been embolized before the others or where newly injected embolic agent has accumulated.
Usually a “roadmap” technique can be used. In other words, a native mask is recorded, which is subtracted from the subsequent live X-ray images, i.e. during the intervention, in order to show changes. If a continuous process is to be viewed, this mask may need to be constantly re-initialized manually.
DE 10 2007 024 450 A1 discloses a method for computing a color-coded first analysis image. A computer receives a temporal sequence of X-ray images, each of which is assigned an acquisition time instant and each of which represents a given contrast-agent distribution in the examination area of an examination object at the respective acquisition time instant. The examination object comprises a vascular system and tissue supplied with blood via the vascular system. The computer computes a characteristic value for each pixel of an analysis image, which corresponds uniquely to one of the blood vessels of the vascular system (single-vessel pixel), for each of the x-ray images, from the data values of the pixels of the respective x-ray image that lie in a first analysis core that is defined by the respective single-vessel pixel and has the same spatial position in all the X-ray images. The x-ray images and the first analysis image correspond spatially with each other pixel by pixel. For each single-vessel pixel, the computer uses the variation over time of the characteristic values of the respective single-vessel pixel to calculate a characteristic time instant for the arrival time of the contrast agent at the respective single-vessel pixel. In addition, it assigns each single-vessel pixel a color attribute that is characteristic of the respective characteristic time instant and assigns every other pixel a color attribute that does not depend on the characteristic time instant. The computer outputs to a user the analysis image color-coded in this manner. A specific color coding is proposed here for each mask re-initialization, but any continuity in the color coding is not reflected.